The present invention relates generally to a vehicle steering control system, and more particularly to a motor driver circuit for an electric steering control system.
In the case of a so called xe2x80x9csteer-by-wirexe2x80x9d system for a motor vehicle, an electric motor actuator is provided for generating torque and applying the generated torque to a rack or linear steering member for steering of a vehicle. In the steer-by-wire system, there is no mechanical connection between the operators steering wheel and the motor actuator. The electric motor actuator generates the torque required to turn a vehicle""s road wheels. However, in an electrical power assist steering system (referred to as EPAS), there is a mechanical connection between the steering wheel and the motor actuator wherein the motor actuator assists a driver""s applied torque. A precise level of generated electrical current is essential to enable the motor actuator to produce an appropriate magnitude of torque for either a steer-by-wire or an electric power assist steering system. The magnitude and direction of torque generated by the motor actuator is a function of a number of variables including a steering wheel angle input, the vehicle""s road wheel angle, and an electric motor actuator current signal which is generated by a motor driver circuit. Although generally not compensated for, the appropriate magnitude of applied torque is also affected by factors such as temperature fluctuations, component-to-component variations, wear of components, and other factors. In an effort to more precisely match motor current to the desired torque in view of the aforementioned variables, designers of steer-by-wire systems have introduced devices which measure the amount of electric current generated by the motor driver circuit and which compensate the measured current by a predetermined value or xe2x80x9coffsetxe2x80x9d.
However, in the above-mentioned prior art steering systems, no compensation is provided for environmental factors such as temperature and other sources of errors in the applied current which can change over time. The accuracy of the current compensation is thus severely limited in these systems. The error in compensation causes an inappropriate amount of current to be applied to the motor actuator resulting in either too much or too little steering torque being applied. There is therefore a need for a steering system that will take into account environmental factors, and other variables including time dependent variables when compensating the current applied to a motor actuator for steer-by-wire and EPAS systems.
It is a general object of the present invention to provide an improved steering control system in which the above-mentioned problems are addressed. A more specific object of the present invention is to provide a steering. control system which continuously monitors a control signal and an electric current applied to the motor actuator so as to improve the quality of current compensation in a steer-by-wire or EPAS system.
In order to achieve the above mentioned objects, there is provided according to the present invention a steering control system for a vehicle having an electric motor actuator, a motor driver circuit for generating and applying an electric current from the motor driver circuit and generating a feedback current signal. The steering control system includes a steering wheel angle sensor for sensing a steering wheel angle and generating a steering wheel angle signal; a vehicle road wheel angle sensor for sensing a vehicle""s road wheel angle and generating a vehicle road wheel angle signal; a controller for setting an offset current signal and for processing the feedback current signal. The steering wheel angle signal, and the road wheel angle signal are used to generate a control signal for the motor driver circuit. The controller compares the control signal to a predetermined threshold value and subtracts the offset current signal from the measured electric current which is applied to the motor actuator when the control signal is greater than the predetermined threshold value. The offset current signal is set equal to an initial offset value when the control signal is less than the predetermined threshold value.
According to the present invention, the controller receives the generated steering wheel angle signal, road wheel angle signal, and feedback current signal to generate the control signal. The control signal is applied to the motor driver circuit and the motor driver circuit generates an electric current which is applied to the motor actuator. The current sensor measures and generates the feedback current to the controller. The controller continuously monitors the control signal and electric current to determine an adequate amount of compensation for the steering control system. Thus, the compensation of the electric current is consistently updated to reflect changes in the steering system during operation.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.